A 3-month-old two-story brick house has a repeating one-at-a-time milisecond sound similar to a large drop of water onto a hard but resonating surface or a wood-on-wood low-pitched tap or creak. It repeats as quickly as two seconds or at an interval as long as 20 seconds and has been fairly constant for about three weeks. It seems to be coming from a first floor wall in a bathroom just behind the toilet and from the continuation of the same wall into a closet in the adjoining room. The wall is an exterior wall and contains copper water pipes and an exhaust vent over the toilet. The weather has been in the 30's and 40's. Please help.